Method of manufacturing a piezoelectric element

ABSTRACT

A COAXIAL CABLE WHOSE DIELECTRIC CONSISTS OF CERAMIC PIEZOELECTRIC MATERIAL. WHEN A CABLE OF THIS KIND IS LOADED MECHANICALLY ON AN ARBITRARY POINT OR OVER A GIVEN DISTANCE, A VOLTAGE CAN BE MEASURED AT THE ENDS BETWEEN THE CENTRAL CONDUCTOR AND THE ENVELOPE. THE CABLE MAY ALTERNATIVELY BE USED FOR GENERATING VIBRATIONS.

United States Patent [191 Baldy June 28, 1974 1 METHOD OF MANUFACTURING A PIEZOELECTRIC ELEMENT [75] lnventor: Jean Baldy, Saint-Cyr-LEcole,

France [73] Assignee: U.S. Philips Corporation, New

York, N.Y.

[22] Filed: Feb. 26, 1973 21 Appl. No.: 335,943

Related [1.5. Application Data [62] Division of Ser. No. 184,695, Sept. 29, 1971,

abandoned.

[52] US. Cl 29/2535, 29/420, 310/82, 310/87, 3l0/9.6, 340/10 [51] Int. Cl B01j 17/00 [58] Field of Search 29/2535, 420; 310/8, 8.2, 310/83, 8.7, 9.6; 340/8 R, 10, 17

[56] References Cited UNITED STATES PATENTS 2,900,536 8/1959 Palo 310/9.6

3,273,146 9/1966 Hurwitz, .lr. 310/83 UX 3,474,268 10/1969 Rudnick 3l0/9.6 X 3,564,304 2/1971 Thorn et al. 310/82 Primary Examiner-Charles W. Lanham Assistant Examiner-Carl E. Hall Attorney, Agent, or FirmFrank R. Trifari; Carl P Steinhauser ABSTRACT A coaxial cable whose dielectric consists of ceramic piezo-electric material. When a cable of this kind is loaded mechanically on an arbitrary point or over a given distance, a voltage can be measured at the ends between the central conductor and the envelope. The cable may alternatively be used for generating vibrat1ons.

1 Claim, 2 Drawing Figures PMENTEHJUN 28 I974 ---m AP(kg/cm METHOD OF MANUFACTURING A PIEZOELECTRTC ELEMENT This is a division of application Ser. No. 184,695, filed Sept. 29, 1971, now abandoned.

The invention relates to a piezo-electric element which may be used, for example, for detecting and measuring pressure variations, vibration phenomena, generating ultrasonic vibrations and the like. The invention also relates to a method of manufacturing such an element.

An object of the present invention is to provide an element which has a wider field of application than the known piezo-electric elements.

According to the invention a piezo-electric element satisfying this object is characterized in that it consists of at least one electrical conductor located within an envelope of electrically conducting material, the space between the conductor and the envelope comprising a ceramic piezo-electric material which is radially polarized.

In its simplest form the piezo-electric element according to the invention consists of one central conductor the axis of which coincides with the axis of a cylindrical envelope. Such an element in the fonn ofa coaxial cable may have an arbitrary length. The commercially available ceramic piezo-electric materials may be used such as, for example, barium titanate doped with lead (for example comprising 5 percent by use of lead titanate) or mixtures of lead titanate and lead zirconate in the form of solid solutions, the so-called lead titanate-zirconate in a modified or non-modified form.

These materials generally have a negative temperature coefficient of the resistance. It is obvious that this is to be taken into account when polarizing at the temperature in the vicinity of the Curie point as will be described hereinafter. A voltage difference between the central conductor and the envelope can be measured at the ends of a cable piece when the cable is mechanically loaded on an arbitrary point.

United States Patent specification No. 3,339,164 describes temperature sensitive elements which have the same structure as the elements according to the invention and in which the space between the envelope and the central conductor is filled up with a material having a negative temperature coefficient. Among the materi' als mentioned in this patent specification there are also materials having piezo-electrical properties. Theelements according to the United States Patent specification No. 3,339,l64 are, however, not polarized radially or in any other manner so that these elements do not have any piezo-electric properties. Neither can it be concluded from this United States Patent Specification that by radial polarisation elements having properties which are principally different from the properties of these elements and consequently have a different field of application can be obtained.

The electrically conducting material to be used for the core and the envelope preferably consists of a metal which, in contact with the piezo-electric material, does not spoil the piezo-electric properties in case of hightempcrature treatments. When using ceramic piezoelectric material which consists entirely or partly of a titanate, these are, for example, the metals titanium, copper and nickel.

The invention also relates to a method of manufacturing the elements according to the invention. This method of manufacturing a piezo-electric element is characterized in that at least one electric conductor is placed within a cylindrical casing of electrically conducting material, a ceramic piezo-electric material being provided in the free space between the electric conductor and the casing, the diameter of the assembly thus obtained being reduced by a mechanical reduction process, the cable-like element then obtained being heated at a temperature in the vicinity of the Curie point of the piezo-electric material, a voltage serving for polarizing the piezo-electric material being applied between the electric conductors and the casing and the material being subsequently cooled.

In this method it is possible to use the known techniques of reducing the cross-sectional surface of composite bodies having a structure in which an electrically insulating ceramic material is provided between a central conductor and an envelope. These operations may consist of rolling, drawing or swaging or of a combination of these operations. The filling may consist of powder, beads or pellets which are slid on the central conductor. If desired, the assembly may be annealed in a suitable atmosphere between the various operations. After the mechanical reduction has taken place the element is brought to a temperature in the vicinity of the Curie point of the ceramic material used. A direct voltage is applied between the casing and the central electric conductor such that the piez-o-electric material is polarized. This polarisation is directed radially. When determining the value of the voltage to be applied, the following factors are to be taken into account: When the polarisation voltage is applied the electrical field between the casing and the central conductor has its maximum value in the vicinity of the central conductor. The ceramic piezo-electric materials have a negative temperature coefficient of the resistance. When heating up to the temperature at which polarisation will take place the resistance of the ceramic material thus decreases. It may be concluded therefrom that those ceramic piezo-electric materials are preferably to be used which have their highest electrical resistance at the Curie point, for these materials admit the highest field strength during polarizing. In this connection the piezo-electric material previously mentioned as examples have been found to be very suitable.

The invention will be described with reference to the accompanying drawing, in which:

FIG. I is a cross-sectional view of an element in the form of a coaxial cable according to the invention and FIG. 2 graphically shows the relationship between different pressure variations and the voltage differences generated thereby in an element according to the invention which is subjected to these pressure variations.

The element according to FIG. It comprises a core I formed by a central conductor whose axis coincides with the axis of the cylindrical envelope 2 which likewise consists of conducting material. A ceramic piezoelectric material which is radially polarized is present between the envelope 2 and the core 1. This is shown in the conventional manner in the Figure by means of radially directed lines provided with and signs which represent aligned dipoles.

EXAMPLE A central conductor of titanium (diameter 3.5 mm) was introduced into a shaft of a titanium tube (external diameter l3 mm, internal diameter mm). The space in the tube was filled up with a ceramic piezo-electric material consisting of a strontium containing lead titanate-zirconate being commercially available (Sn -P- o.9s( 0.41 o.53) a)- The diameter of the assembly was mechanically reduced to an element having an outer diameter of 2 mm, an inner diameter of 1.54 mm of the envelope 2 and a diameter of 0.61 mm of the core 1. The element obtained in this manner was brought to a temperature of 300C. A direct voltage of 500 Volts was then applied between the envelope 2 and the core 1. While this voltage was maintained, the element was cooled to ambient temperature. The piezo-electric properties of the element obtained in this manner were found to be of a permanent nature at temperatures of between and +70C.

An element of this embodiment having a length of 70 cm was placed in an oil-filled tube. A pressure was applied to the oil. The pressure was subsequently decreased rapidly. The maximum occurring voltage difference between envelope and core was measured at different pressure variations with the aid of an oscilloscope having a known impedance. FIG. 2 in which the pressure variation in kg/sq.cm is plotted on the horizontal axis and the maximum/voltage difference is plotted on the vertical axis shows that the signal obtained in this manner is proportional to the pressure variation. The piezo-electric elements in the form of coaxial cables according to the invention may be used inter alia for measuring and detecting pressure variations, vibration phenomena and the like, for example, when testing the vibration level of machines, traffic control, and submarine detectors. The cable may also be used for gen- 4 erating vibrations, for example, ultrasonic vibrations and to this end a suitable electric alternating field is applied between the central conductor and the envelope.

A piezo-electric element according to the invention particularly has the following advantages: it is flexible and may therefore be wound, for example, to a coil. By suitable choice of the outer casing an element may be obtained which is resistant to corrosion to a greater or lesser extent. Due to its shape it may be used for measuring and detecting phenomena whose position is not pre-determined such as in traffic flow counts in which a cable is located at right angles to the travel direction of the vehicles to be counted. Due to its structure the element is better resistant to mechanical influences than the usual elements consisting of bodies of ceramic material provided with electrodes. Finally, due to its radial polarisation it is indifferent for measuring and detecting from which direction the vibration phenomena to be measured or detected is observed.

What is claimed is:

l. A method of manufacturing a piezo-electric element comprising the steps of placing at least one electric conductor within a cylindrical casing of electrically conducting material, filling the space between conductor and the casing with a ceramic piezo-electric material, mechanically reducing the diameter of the assembly thus obtained, heating the assembly to a tempera ture in the vicinity of the Curie-point of the piezoelectric material, applying a voltage between the electric conductor and the casing to polarize the piezoelectric material, and thereafter cooling the material. 

